Humans have been measuring time for a relatively short period in our long history. The desire to synchronize our activities came about 5,000 or 6,000 years ago as our nomadic ancestors began to settle and build civilizations [source: Beagle]. Before that, we divided time only into daylight and night, with bright days for hunting and working and dark nights for sleeping. But as people began to feel the need to coordinate their actions, to be prompt for public gatherings and such, they needed a unified system of keeping time.
Of course, scientists will tell you that we're fooling ourselves if we think we're really keeping track of time. Albert Einstein said "the distinction between past, present and future is only a stubbornly persistent illusion" [source: AMNH]. His daily walks past the clock tower in Bern, Switzerland, led him to some world-changing ideas on the nature of time that we'll look at later.
Whether time is real or not, the measuring of time has become vital to our lives. Over the centuries, people have come up with some creative methods of timekeeping, from the most basic sundials to the cesium-powered atomic clocks of today. In this article, we'll take a look at a variety of methods, including one from China that uses your sense of smell to tell you the time and others that are as old as time itself.
Ancient people turned to nature for the first timekeeping. People began to track the movement of the sun across the sky, and then began to use objects to measure its progress.The Egyptians seem to be the first civilization that made timekeeping a science. They built obelisks -- think the Washington Monument -- about 3500 B.C. and placed them in strategic spots where they would cast shadows. At first, these obelisks only enabled Egyptians to note when it was noon, but they later marked time into further subdivisions [source: Beagle].
Two thousand years later, the Egyptians developed the first sundials that were divided into 10 parts with two twilight hours. A sundial works by tracking the sun's movement. A blade casts a shadow that points to a marked number on the sundial's face, kind of like a clock with only an hour hand. At mid-day, someone has to turn these sundials 180 degrees to measure the afternoon hours. Of course, an ancient sundial could not tell the time on a cloudy day or at night. It was also inaccurate because the sun is at different angles at different times of the year; hours were longer or shorter, depending on the season. Still, a sundial was better than nothing, and by 30 B.C. more than 30 different types were in use in Greece, Italy and Asia Minor [source: Beagle]. Even today, the sun is at the root of our timekeeping system. We've set up time zones ringing the planet to mimic the Earth's rotation around the sun.
Our time-obsessed friends, the ancient Egyptians, seem to have developed the first method of keeping time at night by inventing the first astronomical tool, the merkhet, about 600 B.C. [source: Beagle]. A merkhet is a string with a weight on the end used to measure a straight line, much like a carpenter of today uses a plumb bob. Egyptian astronomers would align two merkhets with the North Star and use that to mark a north-south line, or celestial meridian, in the night sky. Time was counted off as certain stars crossed that line.
Stars also can be used to mark the passage of not just hours, but days. This measurement of the Earth's rotation is called sidereal time. When a certain imaginary point among the stars crosses a celestial meridian, this is called sidereal noon. The time from one sidereal noon to another is called a sidereal day [source: Oracle].
8. Hourglass
Any small child who saw "The Wizard of Oz" knows the power of this timekeeper. Who wasn't frightened when the Wicked Witch of the West turned that hourglass over and told Dorothy she only had an hour to live? Soap opera fans are familiar with it, too, because of the famous opening line: "Like sands through the hourglass, so are the Days of Our Lives."
The hourglass dates back centuries. It's made up of two glass bulbs -- one atop the other -- with a narrow opening between. Sand runs from the top bulb into the bottom when the hourglass is upended. When the top bulb empties, time is up, but it may not necessarily be an hour.
Despite its name, an hourglass can be constructed to measure almost any short period of time by altering the size of the opening between the bulbs or changing the amount of sand in the glass [source: Oracle]. Wealthy people used to proudly display large ornate hourglasses, but hourglasses weren't limited to the rich. Your own grandmother probably had a tiny hourglass egg timer that helped her to cook Grandpa's perfect three-minute egg.
So does a watched hourglass ever empty? This seemingly impatient youngster is closely examining one of man's oldest time measurement techniques -- sand through an hourglass.
Image Credit: Able Images/Getty Images
7. Water Clocks
Water clocks, known as clepsydras -- Greek for "water thief" -- were among the first devices that didn't use the sun or stars to calculate time, meaning they could be used at any time of the day [source: Beagle]. Water clocks work by measuring water that drips from one container to the other. They were invented in Egypt, but their use spread throughout the ancient world and people in some countries still had water clocks well into the 20th century. The ancient Greeks and Romans built impressive towers to house their water clocks, and in China water clocks were called "lou" and often were made of bronze [source: Cultural China]. However, although water clocks were imposing structures, they weren't very accurate.
6. Mechanical Clocks
In Europe during the 1300s, inventive souls began to make mechanical clocks that ran on a system of weights and springs [source: Time]. These first clocks had no hands or faces -- a bell chimed on the hour. In fact, the word "clock" comes from the French "cloche" or "bell" [source: Beagle]. These huge early clocks were installed in churches or monasteries to announce prayers and services. Soon clocks with hour hands began to appear, and later, once the pendulum was refined, another hand was added to keep track of minutes.
The development of springs paved the way for the manufacture of tabletop and mantel clocks. Although more advanced, these timepieces were all notoriously inaccurate. In 1714, the British Parliament offered a reward for anyone who could design an accurate timepiece to aid in sea navigation. The winning watch lost only five seconds over six and a half weeks [source: Time]. With the Industrial Revolution came mass production of timepieces, finally bringing timekeeping into the common man's home.
5. Unusual Clocks
When we think of a clock, we picture the familiar face with numbers, two hands and maybe a sweeping second hand, but that picture is too narrow. Over the centuries, people have developed all manner of machinery to tell time. The Chinese invented the incense clock between 960 and 1279, and its use spread throughout eastern Asia [source: NAWCC]. In one type of incense clock, metal balls were attached to the incense with string. When the incense burned up, the ball would drop, sounding a gong that announced the hour [source: Cultural China].
Other clocks used color, and some used different scents at different times [source: NAWCC]. Candle clocks had numbered markings down their lengths. When the candle burned down to a mark, the dial noted the time. Sometimes the lines were not numbered, and the person using the candle clock would need to know how long it took the candle to burn down to each mark [source: NAWCC].
This 1948 candle clock's shadow climbed up the time dial as the candle burned down.
Image Credit: Orlando/Three Lions/Getty Images
4. Wristwatches
The discovery in the 1400s that coiled springs could move the hands of a clock made smaller timepieces possible. Pocket watches were the order of the day for men for centuries, and wristwatches were considered jewelry -- but for women only. All of those fashion rules changed during World War I, when taking out a pocket watch was impossible during battle [source: Time]. For most of the remainder of the 20th century, almost everyone -- especially men -- wore wristwatches. The gift of a watch symbolized your passage into manhood, and many companies presented you with a gold watch upon your retirement.
As we advance through the 21st century, the ubiquitous wristwatch may once again be fading from style and use. While at work, we can check the time on the computer's clock, and while we're out, our cell phones or MP3 players display accurate times around the clock. Still, an informal survey by a "Houston Chronicle" reporter showed that many people say they will never give up their wristwatches [source: Chronicle].
3. Quartz Clocks
Springs or weights operate the gears inside a mechanical clock, but when the quartz clock came along, electricity of a sort began to move the hands. The mineral quartz -- usually with the help of a battery -- powers a quartz clock. Quartz is piezoelectric, meaning that when a quartz crystal is squeezed, it generates a small current of electricity, which causes the crystal to vibrate when the current passes through. All quartz crystals vibrate at the same frequency. Quartz clocks use a battery to create the crystal vibration and a circuit to count the vibrations. The circuit uses this information to generate one pulse every second. These pulses power the gears in a mechanical clock or power the display in a digital clock [source: NAWCC]. Quartz clocks still dominate the market because of their accuracy and the low production cost [source: Beagle].
Is this worker hiding from the time or her boss? Most offices have a quartz clock on the wall, but time-keeping cell phones and laptops make them unnecessary.
Image Credit: Comstock Images/Thinkstock
2. Atomic Clocks
Although the name sounds rather menacing, atomic clocks are not dangerous at all. They measure time by tracking how long it takes for an atom to switch its energy state from positive to negative and back again [source: NAWCC].
The official time standard for the United States is set by NIST F-1, the cesium atomic clock at the National Institute of Science and Technology (NIST) in Boulder, Colo. NIST F-1 is a fountain clock, named for the movement of atoms. Scientists introduce cesium gas into the clock's vacuum chamber and then direct infrared laser beams at 90-degree angles to the center of the chamber. The lasers force the atoms together into a ball, which gets tossed through a microwave-filled area. The scientists measure the number of atoms with altered states and tune the microwaves to different frequencies until most of the atoms are altered. This final frequency is the natural resonance frequency for cesium atoms and constitutes the number of oscillations that define a second [source: Beagle]. It sounds complicated, but it results in a worldwide standard for seconds; clocks around the world can automatically set to the NIST standard by time zone.
The atomic clock keeps track of time on the most miniscule level, and calendars help us cope with larger blocks of time. Finally, we'll look at how this system of calculating days and years works.
1. Calendars
As we've seen, the actual counting of minutes and seconds requires some complex procedures and machinery, but the tallying of days and months is based on something anyone can observe -- the position of the sun and moon. Different cultures use different methods.
The Christian, or Gregorian, calendar, the one in most common use today, relies on the sun. The Islamic calendar uses the phases of the moon. The Jewish and Chinese calendars rely on a combination of both methods [Source: Harvard].
In the Gregorian calendar, a day is the time from one sunrise to the next, or one complete rotation of the Earth on its axis. A month is about 29.5 days, which is one complete cycle of the moon's phases, and a year is 364.24 days, the time it takes for the Earth to orbit the sun.
For more on measuring time now and throughout history, peruse the links on the following page.
The Aztec calendar stone was unearthed in Mexico City. They cyclic design relates space to days and months.
Image Credit: Victor Chavez/Getty Images
American Museum of Natural History. "Time." (Accessed 09/09/2010). http://www.amnh.org/exhibitions/einstein/time/index.php
Anderson, Carl. "Telling Time without a Clock: The Scandinavian Daymarks." August 18, 1998. (Accessed 09/09/2010). http://hea-www.harvard.edu/ECT/Daymarks/January 7, 2010. (Accessed 09/09/2010).
German Connection. "Punctuality." (Accessed 09/09/2010). http://www.autrata.com/expat/punctuality.html
Levine, Robert. "A Geography of Time: The Temporal Misadventures of a Social Psychologist." Basic Books. 1997.
National Watch and Clock Museum. "How Does It Work? Candle Clocks." (Accessed 09/12/2010). http://www.nawcc.org/index.php/just-for-kids/about-time/how-does-it-work
University of Tennesse, Knoxville. "Timekeeping." (09/09/2010). http://csep10.phys.utk.edu/astr161/lect/time/timekeeping.html
Wallace, Gary. "Why are There 60 Minutes in an Hour?" February 10, 2009. (09/12/2010). http://scienceray.com/mathematics/applied-mathematics/why-are-there-60-minutes-in-an-hour/
Webexhibitis. "Calendars Through the Ages." (09/09/2010). http://www.webexhibits.org/calendars/year.html
Wilkie, Donald M., et. al. "Properties of time-place learning by pigeons, Columbia livia." Behavioural Processes. February, 1994. (09/13/2010). http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T2J-45XSNY0-2J&_user=10&_coverDate=02%2F28%2F1994&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_searchStrId=1459190838&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=587b86b6f6bd18cc558c0c3faed1828c&searchtype=a
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